The present invention relates to apparatus and method for introducing controlled amounts of selected impurity atoms into a surface of a target.
As the number of VLSI devices, integrated with a drastically reduced size on a single chip, has been increasing, the thickness of a gate insulating film for an MOS transistor has been tremendously reduced these days. As for a device with a design rule of 0.5 xcexcm, the thickness of a gate insulating film was once ordinarily set at about 10 nm. In a device now available with a design rule of 0.25 xcexcm, however, the thickness of the film is almost halved, i.e., about 5 nm. A gate insulating film with such a very small thickness is extremely sensitive to charge buildup damage caused by implantation of dopant ions into the gate electrode of the device.
Recently, in order to process a wafer of a much greater size satisfactorily and to increase a throughput sufficiently, the maximum density of a beam current for an ion implanter has been increased. However, if the density of a beam current, created during ion implantation, is increased, then a positive charge buildup phenomenon, resulting from an impacting ion beam, gets even more remarkable. Accordingly, to suppress such a phenomenon, the ability of supplying electrons should be improved for an electron supply system.
FIG. 10 illustrates a cross-sectional structure for a conventional impurity introducing apparatus. As shown in FIG. 10, a guide tube 12 is provided to face a wafer 11 held by a wafer holder 10. A tube bias is applied from a first voltage supply 13 to the guide tube 12. An ion beam 14, which has been generated by an ion beam generator (not shown), travels inside the guide tube 12 leftward to impinge on the surface of the wafer 11.
An arc chamber 16 is provided beside the guide tube 12 and includes a filament 17 therein. A filament voltage is applied from a second voltage supply 18 to both terminals of the filament 17. An arc voltage is applied from a third voltage supply 19 to between one of the terminals of the filament 17 and the arc chamber 16. And arc current is supplied from a current source 20 into the arc chamber 16.
Argon (Ar) gas, for example, is introduced from a gas feed system 21 into the arc chamber 16. By supplying the Ar gas or the like into the arc chamber 16 and applying respective predetermined voltages to the arc chamber 16 and to the filament 17, plasma is created inside the arc chamber 16. And electrons, included in the plasma created, are supplied into the guide tube 12 to have a certain energy distribution.
The guide tube 12, first, second and third voltage supplies 13, 18, 19, arc chamber 16, filament 17, current source 20 and gas feed system 21 constitute an electron supply system 22 for supplying electrons to be introduced into the wafer 11. The electrons 23, which have been supplied from the arc chamber 16 into the guide tube 12, are attracted to a positive ion beam 14 to be distributed around the beam 14 and introduced into the wafer 11 together with the beam 14. The other electrons, which have not been attracted to the vicinity of the ion beam 14, are also attracted and introduced into the wafer 11 by an electric field formed between the guide tube 12 and the wafer 11.
However, the present inventors found that, if ions were implanted into a gate electrode on the wafer 11 using this conventional impurity introducing apparatus, the smaller the thickness of a gate oxide film, the higher the percentage of dielectric breakdown caused in the gate oxide film.
FIGS. 11(a) and 11(b) illustrate relationships between the thickness of a gate oxide film and the percentage of breakdown caused in the film, in which ions are implanted into the gate electrode of an MOS transistor, exhibiting antenna effect, using the conventional impurity introducing apparatus. Herein, the xe2x80x9cantenna effectxe2x80x9d is a phenomenon that if the area of a gate electrode is set larger than that of a gate electrode actually formed in a transistor, then a gate insulating film is affected by the charge of ions and electrons to a higher degree. FIGS. 11(a) and 11(b) illustrate results obtained by implanting the ions into p- and n-type semiconductor substrates, respectively. In this case, As+ ions are implanted under the conditions that accelerating voltage is 20 keV, implant dose is 5xc3x971015/cm2 and a beam current is 10 mA. In the MOS transistor, the area of the gate insulating film is 1xc3x9710xe2x88x926 mm2, the area of the gate electrode is 1xc3x9710xe2x88x921 mm2, and the antenna ratio is 1xc3x97105.
In FIGS. 11(a) and 11(b), the abscissas indicate thicknesses of the gate oxide film, while the ordinates indicate percentages of breakdown caused in an MOS transistor exhibiting the antenna effect, where the breakdown voltage of the gate oxide film thereof is 8 MV/cm or less. FIGS. 11(a) and 11(b) also illustrate respective results obtained with the flux of electrons supplied from an electron supply system varied, where the respective fluxes of electrons are represented as 0.5, 1.0, 1.5 and 2.0 by normalizing the flux of electrons supplied under standard conditions at 1.0. As can be understood from FIGS. 11(a) and 11(b), the smaller the thickness of the gate oxide film, the higher the percentage of breakdown caused in the gate oxide film, even though the flux of electrons supplied from the electron supply system 22 remains the same.
A prime object of the present invention is preventing the percentage of breakdown caused in a gate insulating film from increasing even if the thickness of the film is reduced.
To achieve this object, an apparatus for introducing an impurity according to the present invention includes: means for introducing an impurity having charges into a target to be processed, which is a semiconductor substrate or a film formed on the substrate; means for supplying electrons into the target to cancel the charges of the impurity; and means for controlling the maximum energy of the electrons supplied by the electron supply means at a predetermined value or less.
The apparatus of the present invention includes the means for controlling the maximum energy of the electrons, supplied by the electron supply means, at a predetermined value or less. Accordingly, it is possible to prevent the target to be processed or the semiconductor substrate, on which the target is formed, from being negatively charged up.
In one embodiment of the present invention, the impurity introducing means is preferably means for implanting ions as the impurity.
In such an embodiment, it is possible to prevent a negative charge buildup phenomenon from being caused during the ion implantation.
In another embodiment of the present invention, if an insulating film with a thickness of t (nm) is formed on the semiconductor substrate, then the predetermined value is preferably 2 t (eV).
In such an embodiment, it is possible to prevent dielectric breakdown from being caused in the insulating film due to the negative charge buildup phenomenon.
In still another embodiment, the apparatus preferably further includes means for measuring the energy of the electrons supplied by the electron supply means.
In such an embodiment, the energy of the electrons supplied by the electron supply means can be known.
In such a case, the energy measuring means preferably includes means for measuring the maximum energy of the electrons supplied by the electron supply means.
Then, it is easier to control the maximum energy of the electrons, supplied by the electron supply means, at the predetermined value or less.
In an alternate embodiment, the energy measuring means preferably makes the control means control the maximum energy of the electrons, supplied by the electron supply means, at the predetermined value or less based on the measured energy of the electrons.
In such an embodiment, the maximum energy of the electrons, supplied by the electron supply means, can be automatically controlled at the predetermined value or less.
A method for introducing an impurity according to the present invention includes the steps of: introducing an impurity having charges into a target to be processed, which is a semiconductor substrate or a film formed on the substrate; and supplying electrons into the target to cancel the charges of the impurity. The step of supplying electrons includes the step of controlling the maximum energy of the electrons supplied at a predetermined value or less.
In accordance with the method of the present invention, the step of supplying electrons includes the step of controlling the maximum energy of the electrons supplied at a predetermined value or less. Accordingly, it is possible to prevent the target to be processed or the semiconductor substrate, on which the target is formed, from being negatively charged up.
In one embodiment of the present invention, the step of introducing an impurity preferably includes the step of implanting ions as the impurity.
In such an embodiment, it is possible to prevent a negative charge buildup phenomenon from being caused during the step of implanting ions.
In another embodiment of the present invention, if an insulating film with a thickness of t (nm) is formed on the semiconductor substrate, then the predetermined value is preferably 2 t (eV).
In such an embodiment, it is possible to prevent dielectric breakdown from being caused in the insulating film due to the negative charge buildup phenomenon.
In still another embodiment, the method preferably further includes the step of measuring the energy of the electrons supplied in the step of supplying electrons.
In such an embodiment, the energy of the electrons supplied in the step of supplying electrons can be known.
In this case, the step of measuring the energy preferably includes the step of measuring the maximum energy of the electrons supplied in the step of supplying electrons.
Then, it is easier to control the maximum energy of the electrons, supplied in the step of supplying electrons, at the predetermined value or less.
In an alternate embodiment, the step of measuring the energy preferably includes the step of controlling the maximum energy of the electrons, supplied in the step of supplying electrons, at the predetermined value or less based on the measured energy of the electrons.
In such an embodiment, the maximum energy of the electrons, supplied in the step of supplying electrons, can be automatically controlled at the predetermined value or less.